Fix select decoding on e.g. 32-bit ppc process by 64-bit strace.

Added next_set_bit() function which finds the next set bit,
properly taking into account word size of the traced process.
Use it in decode_select() instead of fd_isset().
Also, properly round fdsize up to word size of traced process,
not to strace's word size.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>

1 files changed, 47 insertions, 0 deletions

diff --git a/util.c b/util.c
index 30a7f195..47c8734f 100644
--- a/util.c
+++ b/util.c
@@ -160,6 +160,53 @@ stpcpy(char *dst, const char *src)
 }
 #endif
 
+/* Find a next bit which is set.
+ * Starts testing at cur_bit.
+ * Returns -1 if no more bits are set.
+ *
+ * We never touch bytes we don't need to.
+ * On big-endian, array is assumed to consist of
+ * current_wordsize wide words: for example, is current_wordsize is 4,
+ * the bytes are walked in 3,2,1,0, 7,6,5,4, 11,10,9,8 ... sequence.
+ * On little-endian machines, word size is immaterial.
+ */
+int
+next_set_bit(const void *bit_array, unsigned cur_bit, unsigned size_bits)
+{
+	const unsigned endian = 1;
+	int little_endian = *(char*)&endian;
+
+	const uint8_t *array = bit_array;
+	unsigned pos = cur_bit / 8;
+	unsigned pos_xor_mask = little_endian ? 0 : current_wordsize-1;
+
+	for (;;) {
+		uint8_t bitmask;
+		uint8_t cur_byte;
+
+		if (cur_bit >= size_bits)
+			return -1;
+		cur_byte = array[pos ^ pos_xor_mask];
+		if (cur_byte == 0) {
+			cur_bit = (cur_bit + 8) & (-8);
+			pos++;
+			continue;
+		}
+		bitmask = 1 << (cur_bit & 7);
+		for (;;) {
+			if (cur_byte & bitmask)
+				return cur_bit;
+			cur_bit++;
+			if (cur_bit >= size_bits)
+				return -1;
+			bitmask <<= 1;
+			/* This check *can't be* optimized out: */
+			if (bitmask == 0)
+				break;
+		}
+		pos++;
+	}
+}
 /*
  * Print entry in struct xlat table, if there.
  */